The present invention relates to a powder coating used in a method for forming a coating on the surface of materials used in various fields, in particular, an insulating material, a material having low heat-resistance and to a method for forming a coating using the same.
This application is based on patent application No. Hei 09-251403 filed in Japan, the content of which is incorporated herein by reference.
In recent years, it is well known that a powder coating containing no organic solvents exerts very few bad influences over persons, and the global environments; therefore, it is extremely beneficial. Therefore, use of a powder coating in various fields in which a solvent coating is employed has been suggested.
An ordinary thermosetting powder coating comprises a thermosetting resin and a curing agent. A coating is formed by making the thermosetting powder coatings adhere to the surface of a material to be coated, and then heating it.
Specifically, the coating methods for making the thermosetting powder coatings adhere to the surface of a material to be coated which is ordinarily employed are as follows:
1. Flow Dip Coating PA0 2. Electrostatic Flow Dip Coating PA0 3. Electrostatic Spray Coating
A material to be coated which is heated above the melting point of the powder coatings (usually approximately 300.degree. C.) is made pass through a bath in which the powder coating particles flow by the force of an air flowing out from a madreporite. Thereby the powder coating particles instantaneously melt by heat radiated from the material to be coated, and adhere on the surface of the material. PA1 A material to be coated fitted with a ground wire and passed through a bath in which the powder coating particles charged by the force of an air flowing out from a madreporite, and thereby, the powder coating particles are made to adhere to the surface of the material to be coated by electrical adhesion. PA1 Powder coating particles are charged inside a spray gun or an overhanging portion thereof, and are sprayed onto a material to be coated which is fitted with a ground wire, and thereby the powder coating particles adhere the surface of the material to be coated by the electrical adhesion.
A cured coating is obtained by making the powder coating adhere to the surface of the material to be coated by these coating methods; heating is carried out in a range of 140 to 200.degree. C. for 20 to 60 minutes, and thereby crosslinking a thermosetting resin and a curing agent comprising the powder coatings.
However, the powder coatings adhere to the surface of the material to be coated by an adhesion or an electric adhesion of the melted powder coatings in these coating methods; therefore, these coating methods cannot be utilized for the materials having low heat resistance, and the insulating materials.
The thermosetting powder coatings used in these coating methods are produced by dry blending a thermosetting resin, a curing agent, and if necessary, an additive, melting and kneading at the temperature more than the flow-softening point of the thermosetting resin, cooling, and then crushing.
A latent curing agent which is stable at room temperature and can crosslink with a thermosetting resin at approximately 140 to 200.degree. C., such as dicyandiamide, imidazolines, hydrazines, blocked isocyanates, acid anhydrides, and dibasic acids, is used for the conventional powder coatings produced by the above formation methods.
When the powder coatings are produced using a curing agent quickly are cured with a thermosetting resin at low temperature, for example, 120.degree. C. or less in the above coating methods, a crosslinking reaction, that is, a curing reaction occurs, during the step of melting and kneading in a kneader, and the cured resin adheres to the groove formed at the shaft. Then, materials for powder coatings are slightly stirred by the kneader, and the viscosity of the kneaded materials becomes large because of the crosslinking. As a result, the amount discharged is small, and the productivity thereof becomes low. Moreover, the obtained kneaded materials are polymerized by the crosslinking reaction, so the hardness thereof is high; therefore, a crushing efficiency thereof becomes low. Therefore, when the powder coatings curing at low temperature are produced in the conventional manufacturing methods, the productivity thereof is low. The powder coatings curing at low temperature produced in the conventional manufacturing methods are unsuitable for industrial large-scale production. Moreover, the obtained powder coatings are polymerized by the crosslinking reaction during kneading, so the flow-softening point thereof is high; therefore, flowing properties during melting step are bad. As a result, a problem, such as the surface of the coating after heating being uneven, arises. In addition, the pot life of the obtained powder coatings is short, the crosslinking reaction between a thermosetting resin and a curing agent is proceeded at 25.degree. C. or less; therefore, the problem arises that storage properties thereof is worse arises.
Therefore, a curing agent which has high reactivity, and is cured at low temperature cannot be used with the thermosetting powder coatings obtained by the above manufacturing methods. In general, a latent curing agent which can be crosslinked with a thermosetting resin at 140 to 200.degree. C. is used to the thermosetting powder coatings obtained by the above manufacturing methods. Consequently, when a coating has to be formed by rapidly curing the powder coatings at 120.degree. C. or less on the surface of the material to be coated comprising of a plastic or a raw material having a low heat resistance, such as electronic components, and electrical components, it is difficult to use a thermosetting powder coating.
As described above, the formation of a coating on the materials to be coated comprises of a plastic; an electronic component, an electrical component; an insulated raw material such as a glass, or a raw material having a low heat resistance, using powder coatings, has problems concerning coating, producing the powder coatings, and low storage properties of powder. When a coating is formed on these materials to be coated, a solvent spray coating using a solvent coating comprising a resin, a curing agent, an additive, and an organic solvent is employed.